Various document copier machines have been produced with the capability of reducing the size of copies made from the documents placed on the document glass. Most of these machines, however, have been designed for providing specific discrete reduction ratios, e.g., of 0.75:1 or 0.66:1. Rarely has an attempt been made to provide a document copier with the capability of continuously variable reduction from ratios such as 1:1 to another ratio such as, e.g., 0.647:1. The few attempts that do appear in the prior art, e.g., U.S. Pat. No. 2,927,503 to Zollinger, and U.S. Pat. No. 3,395,610 to Evans, have operated with a flash exposure, rather than a scanning optical system. It is, therefore, an object of this invention to provide the advantages of a scanning system in a continuously variable reduction document copier machine.
Most conventional non-reduction copy machines utilize a rotating photoconductor-bearing drum with a scanning optical system in order to realize economies over a full-exposure system which must necessarily use a flat imaging surface which is mechanically more complex and consumes more space than a simple rotating drum. Additionally, full-exposure systems have higher power requirements to operate document illumination equipment and can temporarily blind a machine operator if the flash is eye observed. Despite these disadvantages, in reduction optics, most prior art systems opt for the full-exposure procedure to take advantage of the simplicity of its concept. For example, one of the complexities of a scan system utilized in a reduction machine is changing the velocity of the scanning carriages relative to the surface velocity of the rotating drum. Such systems exist in the prior art, exemplified by U.S. Pat. Nos. 3,614,222; 3,897,148 and 3,542,467; but those systems are limited to two, three and five discrete reduction ratios respectively, and therefore only two, three or five ratios of velocities. It is, therefore, an object of this invention to provide a drive system for scanning carriages which adjusts the speed of the scan in a continuously variable manner between boundaries.
In addition to the change of scan velocity, in a reduction system, the length of the scan must also change relative to the length of the image laid down on the photoconductor. For example, at 1:1, an 11-inch document is scanned into an 11-inch image area, but at a 0.647 reduction, a 17-inch document is scanned into the same 11-inch area. Thus it is a further object of this invention to adjust the length of scan relative to the length of the image in a continuously variable manner between boundaries.
A significant problem arises in a reduction scan system involving leading edge registration of the image to the image area. It is desirable for mechanical and timing reasons to match the leading edge of the copy paper to the leading edge of the image area. Therefore, if both the document and the copy paper are 81/2 .times. 11 inches, it is necessary to place the leading edge of the image at the leading edge of the image area in order to transfer the entire image to the copy paper. Also, if a document of 17-inch size is placed on the document glass, it must still be squeezed into an 11-inch image area for transfer to an 81/2 .times. 11-inch sheet of copy paper. Therefore, unless overreduction is practiced, the leading edge of the image of the reduced document must also fall on the leading edge of the image area. However, in a scanning system, as already noted, the scan velocity changes relative to the peripheral velocity of the image area on the photoconductor drum for various reduction ratios. Therefore, the scanning carriage starting position must be shifted in time or space so that it begins to scan the document at the same position on the photoconductive surface regardless of scan speed. Consequently, a further object of this invention is to adjust the leading edge of the scan in a continuous manner with the change in reduction ratio such that the leading edge of the image always falls on the leading edge of the image area.
According to optical theory, a reduction ratio calls for a lens position closer to the image than to the object. However, if a lens is shifted from a 1:1 copying position to a reduction ratio, the plane of the image sharpness also shifts (assuming a constant object plane). Therefore, a problem arises for reduction document copier machines where it is desirable to maintain both a stationary object plane and a stationary image plane, as well as maintain image sharpness. This problem has been approached in discrete reduction systems by providing "add" lens at a particular setting to change the focal length of the lens or by rotating a completely new and different lens into place. Obviously, neither of these approaches can be used if a continuously variable system is desired. U.S. Pat. No. 3,395,610 to Evans, mentioned above, apparently attacks the problem by moving a mirror to the center of the larger document, thus establishing a total conjugate length from document to image, and then adjusting the position of the lens to achieve focal sharpness. This approach results in overreduction of the document and therefore limits the range of usable reduction ratios. Therefore, it is another object of this invention to provide a continuously variable reduction ratio in a machine with stationary object and image planes while maintaining focal sharpness regardless of the magnification ratio selected, to produce document images which are not overreduced.